AM Singer: a tiny AM transmitter

[Simon Orr] wrote in to tell us about his AM transmitter prototype that he plans to put into production in a few months. The build is based on an “Easy AM Transmitter” featured in this Instructables article.

Interestingly enough, this device is capable of transmitting in the 100KHz to 480KHz frequencies. The AM band goes from 520 KHz to 1610KHz, so in order to hear this signal, one must actually tune the radio to twice the emitted frequency. This allows one to tune into the harmonic frequency and receive a signal in this range.

Using the harmonic frequency to transmit is an interesting concept by itself. Additionally, the idea that one could build this device with or without the kit in the future should appeal to experienced hackers and those just starting out alike. Check out the “AM Singer” prototype video after the break.

In the opinion of many the need to place it on the drive, is installing it. There are many ways to share information that use applications that are already installed on most computers. in the words of nluelectronics “forcing”, potential customers to go through unnecessary steps to decide if your product is something they want to buy, has to be a bad business practice, many customers are likely to move on. I did use fritzing I’m still moving on because using it didn’t convince me that is a product I’d buy at any price. In the event I was using my netbook, installing it, and deleting it would have put unnecessary wear on the netbook SSD

The seller’s webpage contains an indirect link to the instructable that this is based on, and a link to a document. Also they give a link to goggle docs for the document that will be included in the kit.

Perhaps you could get a junker laptop that doesn’t care about wear and tear on its moving parts. I drive the dumpsters pretty hard this time of year as it is college move in/out time and there are plenty of “craptops” sitting out and even more desktops. Grab two desktop machines and you can simply put the second hard drive in when the first dies from wear and tear. Really. It sounds like you hit the nail on the head about the business end of this “project”.

This is a cool little transmitter, but I wonder if anyone has looked into the legality of it. The FCC (here in the US) keeps a pretty tight leash on most of the lower frequencies. They do allow unlicensed low power transmitters on some frequencies, maybe this falls into that category.

Back when I started with electronics, I made an AM transmitter. I spent some time looking at HAM licensing and regulations, and I think I came to the conclusion that in the AM frequency range you only need a license for radios with output power of more than 18 mW. I’m not sure about other frequencies…

I remember in high school, they had a driver’s ed parking lot where the teacher would sit in a tower, and would talk to us via the AM radio in the car. I believe the antenna ran underneath the parking lot.

I’ve also seen AM used for weather and/or traffic on some highways; and for advertising a particular house for sale.

PS you guys are really gaining my confidence in the new comment policy. Not only did you over achieve and get the nested reports working in record time, but you guys are on the ball removing troll posts. Great job HAD team.

Why do we continuously see people on HaD making dirty transmitters? The fact that you have to tune this via a harmonic tells me that the person is completely irresponsible in regards to polluting the already jammed RF spectrum and that they also have no idea how to properly work with RF design and circuits. Thumbs down.

Operating a transmitter just to receive its 2nd harmonic is a bad idea, unless you have a notch filter tuned to the fundamental. Which can be out of reach of the beginner radio enthusiast…

AM transmitters are really trivial to make. Why not make them right to begin with? The simplest version only needs a crystal, a common NPN transistor, an audio amplifier and a bunch of capacitors… And it’s a lot more stable than what has been presented here.

Thing is trial, and error, is unlikely to teach a beginner the shortcomings of this circuit. Unless they ramp it up to the point is causes problems for others, and there’s a knock on the door by a person who asks, are you operating an AM transmitter? By that time the question is a matter courtesy, so own up. There are many ways to learn, none of them the proper way as far as I’m concerned, not there’s anything intrinsically wrong with any of them either.

A notch filter is usually designed for a fixed frequency. Given that this circuit wants to be tuneable, a notch filter would probably just eat up most of the signal.

A high order lowpass would probably be better, but it still won’t change the fact that you’re transmitting at both the fundamental and 2nd harmonic. And as said above, driving an antenna with a square wave will produce a lot more than just the 2nd harmonic… and it’s pretty much illegal everywhere.

N0LLK: I am genuinely curious, so please don’t think I’m baiting you here, but: what are the odds that this ‘knock on the door’ will occur?

I always see HAMs cautioning against reckless transmitter use/design by saying “the FCC will shut you down and/or fine you.” Does this really happen all that often? I am not a HAM so I honestly have no idea.

Chances are he’s only going to operate this thing for a little while before he gets bored with it anyway.

john: Go to http://www.arrl.org or pick up an issue of QST, their monthly magazine. They have specific columns on FCC activity and every month the FCC tags someone. That’s just on the amateur bands. The FCC also goes after people outside those bands. The RF spectrum is CROWDED and they are listening.

I don’t see a single coil in the schematic. He is driving the antenna with square wave, producing a lot of harmonics. I’m surprised it works at all, taking into account that antenna is not resonant, and there is not a single tuned LC circuit in sight.

On a positive note, circuit bending like this sometimes produces new breakthroughs.

This circuit is simple and cheap, but probably too simple, and too cheap. Sending square waves out to an antenna will work, but it is almost certainly illegal, no matter what the power level used since it will radiate power across lots of radio spectrum. Even with appropriate filtering, you should really understand Part 15 regulations before trying to setup a neighborhood radio station.

If you use the same circuit to power an LED instead of running the output to an antenna, you can send audio via lightwaves, and avoid the risk of running afoul of the FCC or interfering with your neighbors.

If you want to build something a bit more sophisticated that might actually be legal, try:

The use of harmonics is not a new idea in ham radio, I use the 5th harmonic in one of my lo designs but the lo is enclosed in a shield with a butterworth filter. This sort of device is really annoying people do not realize just how far the interference harmonics go.

While the use of harmonics is not new (and sometimes necessary, ala VHF/UHF), your design is one of sound engineering principle. From the look of their site, this is a high school kid who doesn’t truly understand what he’s doing. Using a square wave to transmit is probably the worst idea ever. Is it something to have in your bag of tricks? Sure. But anyone that actually wants to learn something should do it the right way. Just because you CAN do something, doesn’t mean you SHOULDN’T. I’m wondering if he’s ever even seen the frequency spectrum of a square wave. It is just a shame that one of them claims to have a background in Ham Radio and still made this device.

As a ham radio operator. I know what it takes to get a good signal out on that frequency and just by looking at the design it makes me laugh. Guys really nothing to worry about when it comes to output power. Part 15 regulates the field strenght at a certain distance from the antenna. Do a search on the FCC site and your find the values. a dipole antenna at 1000 Khz is 150 Meter long (493 ft) and I don’t see anything like that attached to it and even if you did, it would even come close to part 15 levels.

I’d hate for your design to go “into production” only to have you shut down by the FCC. Investing all that time and money into a design which intrinsically breaks regulations is a bad idea. I’d recommend that you re-think your design and use techniques that have been tested before instead of relying on circuit bending. Good effort, but poor execution.

Also, I have seen the output from the device. It is effectively a sine wave made of an extremely large number of square waves. I’ve seen the output, and the signal is extremely clean.

True, the transmitter does create a lot of harmonics because it is unfiltered, but it only radiates for ten feet at the most before its signal becomes too weak to matter. There is no harm to the RF spectrum at large. It doesn’t even pass through house walls.

The intent of the transmitter is as a toy, and nothing more. It is not designed to be efficient, but rather to be a kit which fathers and sons can build together to teach how to solder.